This invention provides an integrated process using a dual-functional catalyst for producing epoxides of olefins. In particular, it relates to the production of propylene oxide from propylene wherein the hydrogen peroxide intermediate oxidizing or epoxidizing agent is produced in-situ for the concomitant epoxidation of propylene.
Improved methods of producing propylene oxide or epoxide (PO) have long been sought. The current conventional technologies for PO production are based on the catalytic epoxidation of propylene using organic hydroperoxides such as tertiary-butyl hydroperoxide (TBHP) or ethyl benzene hydroperoxide (EBHP). The TBHP and EBHP are generated by the non-catalytic autooxidation of organic substrates (isobutane or ethyl benzene) with oxygen. However, these existing processes have important drawbacks, which involve multiple reaction steps with intervening distillation separations, resulting in high equipment counts and high capital investment costs. The hydrocarbon substrate feed is first oxidized in a first reactor to generate the hydroperoxide intermediate, which is generally then distilled to recover a portion of the unreacted feed and increase the concentration of the hydroperoxide. The concentrated hydroperoxide is then contacted with propylene feed in a second reactor over a suitable catalyst to generate propylene oxide. A series of separation steps must then be conducted, typically by distillation, to recover the propylene oxide product in purified form.
The complexity of the current process is increased by the fact that for every molecule of propylene oxide primary product generated, at least one molecule of a secondary product is generated. In the case of TBHP intermediate, the secondary product is tertiary-butyl alcohol (TBA), while the use of EBHP leads to the formation of acetophenone. In and of themselves these products are not typically desirable as they require further processing with at least two additional reactors and further distillation to generate more desirable products. TBA is generally converted to methyl tertiary-butyl ether (MTBE) in a two-stage process, first by dehydration to form isobutylene, and then reaction with methanol to form MTBE. Acetophenone is first hydrogenated to form methyl benzyl alcohol, which is then dehydrated to form styrene. These additional steps add undesirable capital and operating costs to the conventional propylene oxide process.
In addition to the added cost and complexity, the production of these secondary products adds further difficulty, because on a weight basis these secondary products are produced in greater amount than the desired propylene oxide product. Commercial markets must be found for the secondary products, and the profitability of the propylene oxide plant is highly influenced by the profitability of the markets for MTBE and styrene. The MTBE market, dominated by use in reformulated gasoline, is currently under severe strain because of environmental and health concerns related to its use. The styrene market is highly cyclical, and is too large to be effectively influenced by propylene oxide producers who produce styrene only as a by-product. As a consequence of these various problems with existing propylene oxide processes, considerable research has been directed towards developing alternate processes for propylene oxide production. Generally, this work has sought to develop processes that eliminate the formation of any secondary product along with the desirable propylene oxide.
Another category of propylene oxide processes is based on direct oxidation of propylene with oxygen. For example, U.S. Pat. Nos. 5,698,719; 5,686,380, 5,864,047; 5,625,084; 5,861,519; and 5,763,630 disclose catalysts based on silver for the direct oxidation of propylene to propylene oxide. U.S. Pat. No. 5,703,254 discloses combining silver and gold as catalyst. U.S. Pat. No. 5,760,254 discloses a nitrogen oxide catalyst. U.S. Pat. No. 5,670,674 discloses a platinum-based catalyst. However, none of these patented processes have yet reached a status suitable for commercialization. Generally, the overoxidation of propylene to form carbon oxides such as CO2 is a major problem. A suitable combination of catalyst activity, selectivity, and catalyst life has yet to be achieved, and is likely to present a continuing challenge due to the tendency of molecular oxygen to cause complete oxidation reactions to form CO2.
An alternate approach to a new propylene oxide process is the use of hydrogen peroxide as the oxidizing agent. Unlike organic hydroperoxides such as TBHP and EBHP that form organic by-products during the epoxidation of propylene, the by-product of reacting propylene with hydrogen peroxide is water, an innocuous compound. U.S. Pat. No. 4,701,428 discloses a titanium silicalite catalyst (TS-1), which can be used for the epoxidation of olefins using hydrogen peroxide; this patent is incorporated herein by reference with respect to the titanium silicalite portion of the disclosure. In the epoxidation of propylene to form propylene oxide, selectivity as high as 93% is obtained, based on hydrogen peroxide consumed. Other similar patents are U.S. Pat. Nos. 4,859,785 and 4,954,653. U.S. Pat. Nos. 4,937,216 and 4,824,976 also disclose processes based on TS-1 catalyst for the epoxidation of various olefins, including propylene, and they report selectivities of epoxide formation as high as 98%.
U.S. Pat. Nos. 5,166,372; 5,214,168; 5,262,550; 5,384,418; 5,646,314; 5,693,834; 5,523,426; 5,912,367; 6,066,750 all disclose various versions of an olefin epoxidation process (especially a propylene epoxidation process) where a titanium silicalite is used as the epoxidation catalyst and hydrogen peroxide is used as the epoxidizing agent. Generally, in these patents, hydrogen peroxide is generated in a separate reactor by the autooxidation of a secondary alcohol such as isopropanol. U.S. Pat. Nos. 5,679,749; 6,042,807; and 5,977,009 disclose variations on titanium-based zeolitic catalysts containing other components such as tellurium, boron, germanium, niobium, which are claimed to increase the activity or selectivity of the catalyst for the epoxidation of olefins such as propylene. Also, U.S. Pat. Nos. 5,374,747; 5,412,122; 5,527,520; 5,554,356; 5,621,122; 5,684,170; and 5,695,736 disclose catalysts containing Si and Ti which are isomorphous in structure with the zeolite beta structure. These catalysts are claimed to be useful for the selective epoxidation of olefins such as propylene using hydrogen peroxide as an oxidant.
However, there are significant shortcomings for these prior art processes in the hydrogen peroxide-based epoxidation of propylene that have prevented their commercialization. The cost of hydrogen peroxide produced by current means is generally too high for the peroxide-based route to PO product to be economical. Also, these prior art processes are based on a multi-step approach in which hydrogen peroxide is separately generated using suitable oxidation technology, and then the hydrogen peroxide is used to epoxidize propylene. Normally, there are separation steps provided between these reaction steps. Also, the synthesis of hydrogen peroxide by conventional means normally involves the hydrogenation of a working medium such as anthraquinone or secondary alcohol; this must be oxidized in a third reaction step to regenerate the working medium for re-use in the hydrogen peroxide synthesis.
Another approach is to combine the synthesis of hydrogen peroxide and the epoxidation of propylene into a single step reaction. This requires a dual-functional catalyst, capable of catalyzing the direct reaction of hydrogen and oxygen to form hydrogen peroxide, and simultaneously catalyzing the reaction of said hydrogen peroxide with propylene to form propylene oxide product. Examples are provided by U.S. Pat. Nos. 5,973,171; 6,005,123; 6,008,388; and 6,063,942, all of which disclose catalysts based on combinations of titanium or vanadium based zeolitic structures with noble metals such as palladium. The noble metal constituent provides for the catalytic synthesis of in situ or surface hydrogen peroxide, and the titanium or vanadium-based zeolite catalyzes the epoxidation of propylene by the hydrogen peroxide. However, these prior art processes have also fallen short of requirements for commercialization. The prior art dual-functional catalysts is are not sufficiently selective, and the noble metal constituent is generally not bound strongly enough to the substrate surface to prevent loss of metal surface area through metal leaching to the liquid phase or through sintering of metal particles. Because of the high cost of noble metals such as palladium, this loss of active surface area is unacceptable.
These prior art processes have also failed to utilize the role of crystal-face exposition of the noble metal particles in achieving high selectivity of hydrogen peroxide production as taught in applicants"" U.S. Pat. No. 6,168,775 B1. The selectivity of the hydrogen peroxide catalyst is highly dependent on the crystal face with the 110 and 220 crystal faces being much preferred for selective synthesis of hydrogen peroxide intermediate from hydrogen and oxygen. U.S. Pat. Nos. 4,661,337; 4,681,751; 4,772,458; 4,832,938; 5,236,692; 5,378,450; 5,399,334; and 5,338,531 disclose various catalysts based on the use of noble metals such as palladium and other platinum group metals for direct synthesis of hydrogen peroxide from hydrogen and oxygen. But by failing to properly control the noble metal crystal face exposition, these prior art catalysts generally have selectivities of hydrogen peroxide synthesis based on hydrogen consumed of less than 85%. Because hydrogen is a costly feedstock, this low selectivity leads to costly inefficiency in the process.
In applicant"" U.S. Pat. No. 6,168,775 B1, incorporated herein by reference in its entirety, these problems are addressed for the direct synthesis of hydrogen peroxide from hydrogen and oxygen feeds. In applicant"" patent ""775, an ionic polymer such as sodium polyacrylate is used during the deposition of active noble metal, such as palladium, onto the support or substrate to form a peroxide synthesis catalyst. The function of the ionic polymer is to act as a dispersing and control agent to disperse the metal particles on the surface and control their face exposition. Accordingly, under the conditions taught in the ""775 patent, the desired crystal face of the noble metal particles are selectively exposed. The process of the ""775 patent provides a catalyst with very high selectivity of up to 100% for the direct synthesis of hydrogen peroxide from hydrogen and oxygen feeds. In addition, this catalyst is highly active, giving yields of hydrogen peroxide per weight of noble metal per hour greater than known prior art catalysts, despite being operated at hydrogen feed concentrations of less than the hydrogen flammability limit of 4.5 volume percent in oxygen or 4 volume percent in air.
An object of this invention is to address the foregoing problems of propylene oxide production by propylene epoxidation by providing an integrated or combined process using a unique porous dual-functional catalyst having high activity, stable structure, and long life. The dual-functional catalyst can both selectively catalyze the formation of hydrogen peroxide from hydrogen and oxygen and epoxidize propylene feeds to form propylene oxide product. Preferably, the process is carried out in situ in the same reaction vessel. This objective is accomplished using a dual-functional catalyst consisting of dispersed noble metal nanometer-sized crystallite particles such as palladium (Pd) on a titanium-based zeolitic catalyst substrate. The noble metal particles are deposited using a colloid solution in which an ionic polymer is used for dispersing the particles, thereby creating a high surface area of active noble metal having a controlled face exposition. The desired dispersion and controlled crystal-face exposition is achieved using a critical molar ratio of noble metal to ionic polymer in the broad range of 1:0.1 to 1:10, depending on the molecular weight of the polymer, with a preferred range of 1:0.5 to 1:5. By controlling the crystal face exposition, the selectivity of the catalyst is maximized. This dual-functional catalyst converts the feed streams of hydrogen, oxygen, and propylene into propylene oxide in single reactor, and operates effectively at hydrogen feed concentrations below the lower flammability limit of 4.5%. The only by-product of this reaction is water, which can be easily separated from the propylene oxide product.
The titanium-based zeolitic substrate is chosen based on its capability to catalyze the hydrogen peroxide-based epoxidation of propylene to form propylene oxide. For example, titanium silicalite (TS-1) is a suitable substrate, but other suitable substrates may also be selected as known to those skilled in this art. Any material which is a solid under process operating conditions and can catalyze the epoxidation of propylene to propylene oxide can be used as a catalytic substrate for the dual-functional catalyst of the invention. Useful noble metals for the dual-function catalyst utilized in this invention are palladium, platinum, gold, iridium, osmium, rhodium, ruthenium, and combinations thereof. The useful broad percentage concentration for the noble metal in the catalyst is 0.01 to 10 wt %, with a preferred range of 0.1-5 wt %. Suitable ionic polymers or other complexing-dispersing polymers for making the noble metal crystallite catalyst should be either negatively charged or have a lone pair of electrons that can attract the positively charged metal ions such as Pd2+. Ionic polymers preferably have molecular weights within the range of about 300-8000, and more preferably 600-6000. Examples of suitable polymers include polyacrylates, polyvinylbenzoates, polyvinyl sulfate, polyvinyl sulfonates, polybiphenyl carbonates, polybenzimidozoles, polypyridines, and other polymer agents having similar molecular structures and properties.
In the hydrogen peroxide production and propylene epoxidation process of the invention the dual functional catalyst is prepared in a solvent by depositing noble metal on a suitable substrate in the presence of a dispersing agent. Preferred solvents include water and lower alcohols such as methanol.
The dual-function catalyst of this invention is employed in an integrated process including a suitable reactor containing the catalyst, along with suitable systems for catalyst recovery and recycle, unreacted feedstock recovery and recycle, and product recovery and purification. Hydrogen, oxygen, and propylene are simultaneously and continuously fed into the reactor. This reactor will preferably be a single reactor chamber or vessel, but may also consist of multiple reactors connected in series or parallel. The reactor(s) is operated at a temperature in the range 0-150xc2x0 C. and a pressure in the range 100-3000 psig, preferably 10-100xc2x0 C. and 500-2000 psig. The catalytic reactor may be a fixed bed, slurry, fluidized bed, or other suitable type for contacting the solid catalyst with liquid and gaseous reactants.
The advantages of the invention for the production of propylene oxide product by an integrated process are:(1) a high overall selectivity of propylene oxide formation, greater than 90% with respect to hydrogen feed consumed, and greater than 90% with respect to propylene feed consumed; (2) greatly reduced capital cost because of reduction in major equipment resulting from the single reaction step and the reduced number of separations required to prepare the hydrogen peroxide intermediate; (3) formation of only water as a by-product; (4) safe operation resulting from the hydrogen concentration being maintained below the flammability limit; (5) safe operation owing to the in situ reaction of the peroxide intermediate, eliminating any need to isolate, purify, or otherwise handle peroxide compounds outside of the reactor; and (6) long catalyst life and low loss of active noble metal surface area due to the strong bonding of noble metal particles to the catalyst substrate surface. Because of these advantages, the invention represents a major advance compared to the known prior art processes for propylene oxide production.
In general, the invention comprises a porous particulate dual-functional catalyst for the selective combined in-situ production of hydrogen peroxide from hydrogen and oxygen concurrent with the epoxidation of olefins. The dual-functional catalyst comprises a catalytic substrate material comprising at least one olefin epoxidation catalyst; and at least one crystalline noble metal exhibiting crystallites of nanometer size deposited on a portion of the surface of said substrate. Of particular significance is the fact that the crystallite faces of the deposited noble metal crystals are mainly composed of the 110 and/or 220 series of crystal planes.
Further, the invention includes a method for preparing a porous, dual-functional catalyst comprising nanometer-sized noble metal-containing catalyst crystals deposited on a particulate catalytic substrate for the combined in-situ production of hydrogen peroxide from hydrogen and oxygen concurrent with the epoxidation of an olefins feedstream. The method includes preparing a dilute acid solution containing a noble metal salt including a palladium salt alone or in combination with a minor amount of one or more salts of platinum, gold, iridium, osmium, rhodium or ruthenium; mixing a water-soluble catalyst impregnation control ionic polymer into the dilute acid solution of noble metal salt; reducing the mixed solution of noble metal salt and the impregnation control ionic polymer; adding the particulate catalytic substrate to the reduced mixed solution and impregnating the substrate with the noble metal portion of the reduced mixed solution; recovering and drying the impregnated substrate; and reducing the impregnated substrate with hydrogen to produce the dual-functional catalyst wherein the face of the deposited noble metal crystals are composed mainly of the 110 and/or 220 series of crystal planes.
In addition, a method is disclosed for the epoxidation of olefins simultaneously with the selective in situ generation of hydrogen peroxide. The method comprises concurrently contacting feedstreams comprising hydrogen, oxygen and olefins in a solvent in a reactor vessel containing the particulate dual-functional catalyst described above under reaction conditions sufficient to generate hydrogen peroxide in situ from the hydrogen and oxygen feedstreams while epoxidizing the olefin feedsteam with the in situ generated hydrogen peroxide. A reactor effluent stream is produced containing unreacted gaseous components, particulate catalyst, unconverted liquid olefins, olefin epoxides, solvent and water. The effluent stream is separated to recover the olefin epoxide product and recycle the particulate catalyst, unreacted olefin, unreacted hydrogen, unreacted oxygen and solvent.
While the preferred use of the dual-functional catalyst of the invention is in a process where the hydrogen peroxide production and olefin epoxidation production takes place concurrently, preferably in the same reactor vessel, the process can be carried out sequentially in separate vessels without departing from the scope of the invention. A sequential process of the invention will still enjoy the high selectivity of hydrogen peroxide production at very low hydrogen flammability concentrations but will incur greater vessel counts and similar economic penalties. Thus, a sequential process defeats some of the key advantages inherent in the use of the dual-functional catalyst of the invention for hydrogen peroxide production and olefin epoxidation. The preferred mode of the process of the invention is concurrent in situ production of the aforenoted products.